Different imager systems are well known systems for collecting and processing optical data. Known imager systems include so-called “pushbroom”, “whiskbroom” and “frame” imagers. Within this document “optical data” means electromagnetic radiation in the form of electromagnetic rays having wavelengths ranging from ultraviolet through the infrared.
Pushbroom imagers are widely used in remote sensing instrumentation. Such imagers are typically used in situations where there is relative motion between the area being imaged and the imager. Such motion can be provided by mounting the imager on a vehicle, aircraft or satellite and traversing an area of interest with the imager oriented so that the area of interest passes through the field-of-view of the imager. Alternatively, the imager may be fixed and the area of interest moves through the field-of-view of the imager as, for example, on a conveyor belt. The typical result of imaging is the creation of a strip image, produced by imaging one entire line within the field of view at a time, or possibly a series of contiguous lines, approximately at right angles to the track of the relative motion between the imager and the area of interest.
In comparison, a whiskbroom imager images a single point at a time and scans this point at right angles to the track to build up a line image. A frame imager collects a series of fixed frame two-dimensional images along the track.
A pushbroom imager can utilize one or more sensors in the focal plane of the imager consisting of a linear array of sensing pixels or a two-dimensional array of sensing pixels. Linear or two-dimensional arrays can be referenced generically as “focal plane arrays” (FPA).
Typically, a spectrographic push-broom imager has a narrow slit, usually installed at the image plane of an optical train, such that only a narrow portion of the field of view, typically at right angles to the direction of relative motion between the spectrograph and the area being imaged, is passed through the slit into a second optical train (containing spectrally dispersive optical elements) and onto a focal plane array (usually a two-dimensional focal plane array). This narrow spatial area passing the slit is typically referred to as the “across-track line image”. The two-dimensional focal plane array is typically oriented such that the optical elements of the spectrographic push-broom imager align the across-track line image along one axis of the array and spectrally disperse the light from this image at right angles along the other orthogonal (“column”) axis of the array. Hence, each “row” of the sensor is exposed to light from the field-of-view of the same across-track line image on the ground (or other area of interest) but at a different wavelength. Similarly, each column of the sensor records the spectrum of a given point within the across-track line image.
The spectrally dispersed light energy or optical data of the across-track line image creates a measurable change in information in each exposed pixel of the focal plane array. Typically, the measurable change in information in each pixel or some combination of pixels is read out by the electronics associated with the imager at some desired integration time. The mode of sampling and the length of the integration times can vary according to the details of the specific instrument design and the operational parameters selected for a particular measurement. As multiple, spectrally-dispersed, across-track line images are read out and recorded on suitable recording media, a “spectral image” of the total area viewed by the multiple across-track line images is created. In the context of this description, the definitions of “row” and “column” are a matter of convention and are not relevant to the substance of the invention.
Often, the number of individual spectral values associated with one spatial column of the focal plane array is much smaller than the number of desired spatial columns that define the across-track swath of the imager. Increasing the spectral resolution is accomplished by spreading the available radiation from any given point in the scene over as many different rows as there are spectral bands. A large number of spectral bands results in a weaker signal, such that a tradeoff must be made between spectral resolution and signal-to-noise ratio, which is governed, in part, by the amount of energy striking each pixel. This trade-off usually means that the number of spectral bands desired (rows) is often far less than the available number of rows on typical sensor focal plane arrays along the spectral axis (which is typically chosen to be the smaller of the two dimensions in a rectangular focal plane array).
Thus, the number of across-track pixels available from pushbroom imagers is often limited by the pixel arrangement of commercially available focal plane sensor arrays. Such focal plane sensor arrays are usually designed for two-dimensional scene imaging and tend to have approximately equal dimensions for rows and columns. In contrast, the ideal sensor for imaging spectroscopy would have a very large number of columns (spatial information) compared to its number of rows (spectral information). Thus current designs for push-broom imaging spectrometers can under-utilize the focal plane rows (used for the spectral dimension) and lack the desired number of columns (for the across-track spatial dimension).
This poor utilization of the focal plane array can be mitigated by designing specialized custom sensor arrays. However, specialized custom sensor arrays can only be produced at very high costs that tend to defeat the objectives of providing commercially viable pushbroom imaging instruments and in addition there are often practical constraints as to the maximum dimension of an array due to fabrication limitations.
The net result of this “standard” focal plane geometry is that the desired spatial resolution may be compromised. Compromised spatial resolution will, as a result, increase the required number of separate passes over the scene of interest that are needed to cover an area due to the limited swath width. An increase in the number of passes increases the time and cost to acquire such imagery.
Moreover, even if focal plane arrays with large numbers of spatial pixels were available, the standard design would lead to the need for increased dimensions of the optical components to accommodate the larger image dimension, resulting in substantially greater size and costs of the optics.
Hence, there has been a need for a system that permits more efficient usage of the optical focal plane. In particular, there has been a need for creating a “virtual” focal plane array that has many more across-track spatial pixels (columns) than the number of pixels in the spectral direction (rows) while still employing optics consistent with the original image format. Further still, there has been a need for imaging systems that enable efficient use of system optics that enable the collection of optical data from one or more fields of view wherein the optical data from each field of view is passed through a common optical train in order to minimize the physical size of the optical components.
Similarly, the same problem arises with non-spectrographic pushbroom imagers. In the visible and near-Infrared wavelength range, there is a ready supply of long linear sensor arrays with thousands of pixels. For this wavelength region, these long linear arrays can address the problem as described above for providing a wide (high resolution) swath. However, better sensor availability does not address the problem of the concomitant need for larger optical elements to accommodate these long linear arrays.
For non-spectrographic pushbroom images designed for other parts of the spectrum, such as the short-wave and thermal Infrared, the most cost effective sensors may be two-dimensional arrays with approximately equal numbers of rows and columns. For these wavelength regions, similarly, there has been a need for creating a “virtual” focal plane array that is substantially greater than the maximum physical dimension of the array.
Similarly, although spectrographic imagers typically employ a single slit and then image the slit through wavelength-dispersing optics onto a sensor array, certain types of spectrograph optics can function the same way with multiple parallel slits. In these imagers, multiple spectra are produced in the focal plane which are displaced at right angles to the slits according to the separation of the slits. In this case, it is only necessary that the slits be sufficiently separated to avoid overlap of the spectra. Accordingly, there is also a need for improved imaging systems that enable efficient usage of the optical focal plane in spectrographic imaging systems utilizing multiple slits.
A review of the prior art reveals that past systems have not provided an effective solution to the above problems.
U.S. Pat. No. 5,936,771 describes narrow and wide field of view forward looking Infrared (FLIR) optics with mechanical switching between the different fields of view.
U.S. Pat. No. 6,903,343 describes a system of lightweight laser designator ranger FLIR optics. This is a complex system that divides incoming radiation from a single aperture, and passes it through separate optics onto two different sensors to give wide and narrow fields of view. This patent describes a system in which incoming radiation from a single scene is divided between separate optical systems and does not employ a common set of optics.
U.S. Pat. No. 6,888,141 describes a frame imager that uses a pyroelectric film illuminated by thermal radiation to modulate the reflection of visible light and form an image on a visible light detector array. In this system, incident visible radiation that is less well reflected produces additional heating of the pyroelectric layer, causing positive feedback and increasing the gain of the system.
U.S. Pat. No. 6,774,366 describes an image integration and multiple laser source projection system. The system does not describe an imaging system in which optical data from two or more fields of view is simultaneously passed through a common optical path.
U.S. Pat. No. 5,751,473 describes a dual waveband optical system that uses a dual wavelength quantum well sensor array. In this system a dichroic beamsplitter separates mid-wave infrared (MWIR) and long-wave infrared (LWIR) radiation, passing first through additional optics that increase the focal length and provide a narrow field of view. The beam diversion is internal to the optics rather than separate, and emphasizes providing one narrow field of view within a broader field of view. In addition, this system does not spread the separate fields of view across a single or multiple detectors but rather employs a single dual-wavelength detector.
U.S. Pat. No. 5,414,364 describes an optically multiplexed dual line of sight forward looking infrared (FLIR) system using a chopper to alternate a single sensor array between two separate optical trains having distinct fields of view. As such, the sensor views two fields of view consecutively, alternating between them, thus using temporal rather than spatial multiplexing.
U.S. Pat. No. 5,049,740 also describes a multiple field of view sensor and system that mechanically alternates between narrow and wide fields of view.
U.S. Pat. No. 4,765,564 describes a solid state apparatus for imaging. The patent describes a system that splits the radiation from a single field of view by employing wavelength-dependent filtering. The patent does not describe an imaging system in which optical data from two or more fields of view is simultaneously passed through a common optical path.
U.S. Pat. No. 4,682,029 describes a dual infrared (IR) scanner setup for stereo imaging. The system includes two scanners that alternately illuminate a single detector array.